This invention relates generally to ultrasound imaging, and more particularly the invention relates to a method of estimating near field aberrating delays in pulse echo imaging systems.
In almost any practical pulse-echo imaging system which utilizes phased arrays, the imaging system impulse response tends to widen in the presence of near filed propagation inhomogeneities. In the case of medical acoustic imaging, focusing errors in high-performance systems are primarily a result of inhomogeneities in the near-field propagation medium.
A fair amount of prior art exists on the subject of correcting for near field distortions in diffraction limited phased array systems. The most direct approach involves imaging a target which can be modeled as a single isolated point scatterer. Following the transmission of a pulse from the entire aperture, the signals obtained by each receiver element are virtually identical, apart from a relative lag imparted by aberrating delays. The relative delays between two signals can be estimated by determining the peak in their cross-correlation function. Refinements in this approach to cover moving targets which contain a large point source corrupted by smaller sources have been developed for imaging with radar, B. D. Steinberg, "Microwave imaging of aircraft," Proc. IEEE 76(2), 1578-1592 (1988). In acoustic imaging Flax and O'Donnell have had success in applying the cross-correlation technique while imaging targets composed of diffuse random scatterers, S. W. Fax and M. O'Donnell, "Phase-aberration correction using signals from point reflectors and diffuse scatterers: basic principles," IEEE Trans. Ultrason. Ferroelectr. Freq. Control UFFC-35(6), 758-767 (1988).
Muller's and Buffington's work in optics spawned a number of iterative approaches which use a sharpening function as a criterion for image quality, R. A. Muller and A. Buffington, "Real-time correction of atmospherically degraded telescope images through image sharpening," J. Opt. Soc. Am. 64(9), 1200-1210 (1974). They proposed several classes of sharpening functions, all of which are expressible as integrals whose integrals whose integrands are a function of the image intensity. The sharpening functions are maximized in the absence of focusing aberrations. The investigations of Muller and Buffington centered on incoherent stellar imaging. Gamboa has adapted these approaches to ultrasonic imaging, A. Gamboa, "Ultrasonic imaging through an inhomogeneous medium with a linear phased array," Ph.D. thesis, Stanford University, Department of Electrical Engineering, 1988. His algorithms rely on the presence of a dominant reflector or "glint" in the domain of the sharpening function integral.
In other recent work, Nock et al. have demonstrated success in acoustic imaging when using a sharpness function represented as the integral of the backscatter magnitude of the image data, L. Nock, G. E. Trahey, and S. W. Smith, "Phase aberration correction in medical ultrasound using speckle brightness as a quality factor," J. Acoust. Soc. Am. 85, 1819-1833 (1989). The patented approach generally does not rely on the presence of a dominant reflector, and has demonstrated some success in prototype.
Synthetic aperture techniques have evolved which compensate for near field aberrations. As an example, Hirama et al. recognized that both a phase aberrator and target distribution could be reconstructed from the full set of data acquired by separately transmitting and receiving with each element in an acoustic phased array, M. Hirama and T. Sato, "Imaging through an inhomogeneous layer by least-mean-square error fitting," J. Acoust. Soc. Am. 75, 1142-1147 (1984). They did not develop an approach to cover targets which extend in range, and they did not propose a means of acquiring the information from a single scan.